Device and method for 3D height-finding avian radar

ABSTRACT

A height-finding 3D avian radar comprises an azimuthally scanning radar system with means of varying the elevation pointing angle of the antenna. The elevation angle can be varied by employing either an antenna with multiple beams, or an elevation scanner, or two radars pointed at different elevations. Heights of birds are determined by analyzing the received echo returns from detected bird targets illuminated with the different elevation pointing angles.

FIELD OF THE INVENTION

This invention relates to ground-based radar systems and methods. Theinvention relates more additionally and more specifically to radartarget detection, tracking and estimation of target height. Theinvention is particularly useful in radar surveillance of birds andother airborne targets.

BACKGROUND OF THE INVENTION

Avian radars are used to track birds in flight in the vicinity ofairfields, wind farms, communications towers, and along migrationroutes. Birds are a significant hazard to aviation safety. Applicationsthat require bird monitoring are the bird aircraft strike hazard (BASH)problem and the natural resource management (NRM) problem. Billions ofdollars in damage to aircraft and significant loss of life have beenrecorded due to birds flying into aircraft, particularly during take-offand landing in the vicinity of airports.

The danger associated with birds depends on their altitude (among otherfactors). Users of bird detection and tracking radars need to know theheight of tracked birds. State-of-the-art avian radars provide targettracking with localization in only two dimensions. These systems do notestimate height (within the beam extent) in any real sense. Thus avianradars need altitude estimation of bird (or other airborne) targets.They need the means to estimate target height in a manner that ispractical and economical. The purpose of the current invention is toprovide next generation avian radars with such means, thereby overcomingcurrent limitations in the state-of-the-art.

State-of-the-art avian radars use inexpensive, commercial-off-the-shelf(COTS) X-band marine radar transceivers, fitted with slotted-waveguidearray antennas, as well as parabolic reflector or Cassegrain (dish)antennas. The raw received baseband signals are digitized, followed bydetection and tracking of bird targets. State-of-the-art avian radarsprovide continuous, day or night, all-weather, situational awarenesswith automated detection, localization and warnings of hazards. Theyprovide high-quality target track data with sophisticated criteria todetermine potentially dangerous target behavior, as well ascommunication of alerts to users who as require that information. Theyalso minimize operator interaction.

State-of-the-art avian radars features include:

-   -   Low-cost, high-performance radar antennas and transceivers        mounted on ground-based pedestals    -   Radar processing that can reliably detect and track small,        low-RCS (radar cross-section), maneuvering targets in dense        target and clutter environments    -   Automatic hazard detection and alert capability to remote users    -   The formation of radar networks to provide wide-area coverage    -   Low cost of operation    -   Low life cycle costs    -   Data and analysis support for research and development

COTS marine radars are very inexpensive. These marine radars exhibitsurprisingly good hardware specifications. However, as-is, these radarsdeliver mediocre performance for bird targets because of their primitivesignal processing. Combining a COTS marine radar with a digitizer boardand a software radar processor that runs on a COTS personal computer(PC) and a parabolic dish antenna forms a state-of-the-art avian radar,one with a very limited three-dimensional (3D) localization capability.Modifying such radars via custom antennas and processing allows heightestimation and coverage.

Slotted-waveguide array antennas are used to provide two-dimensional(2D) localization (i.e. range and azimuth, which can be translated tolatitude and longitude). These systems provide good volume coverage dueto the typically larger vertical (elevation) beamwidth, which is on theorder of 20 degrees. Such systems cannot provide useful height estimatesof tracked targets when the radar is spinning horizontally in its usualorientation. This is because the beam uncertainty in the 3^(rd)dimension (elevation), which is on the order of the beam extent, is toolarge. For example, the elevation beam extent or height uncertainty fora target at a distance of just 1 km from the radar is about 1,000 feet.This means that if both a plane and a bird are being tracked by theradar at a distance of 1 km away, the radar cannot tell whether the twotargets are 1,000 feet apart (i.e. one is on the ground and the other isat the upper edge of the vertical beam, 1,000 feet off the ground) orwhether they are at the same altitude where a collision could occur.While some radar configurations orient the slotted-array antenna so thatit spins vertically (rather than horizontally) to get a measure ofheight, see Nocturnal Bird Migration over an Appalachian Ridge at aProposed Wind Power Project, Mabee et al, Wildlife Society Bulletin34(3), 2006, page 683, they still can only operate as 2D radars. Inorder to measure height, they can no longer provide 360-degree azimuthalcoverage (which a conventional azimuth-rotating radar provides).

Parabolic reflector or Cassegrain (dish) antennas are used today toprovide a very limited 3D localization capability. These antennas employa single beam (pencil shaped), fixed in elevation, but rotating inazimuth. The azimuth rotation results in the usual 2D, 360-degreecoverage with localization in range-azimuth or latitude-longitude.However, by using a narrow pencil beam (say between 2 and 4 degreeswide), the height uncertainty reduces significantly as compared to the20 deg slotted-array antenna. Using the previous example, with targetsat a distance of 1 km from the radar and a 4-degree dish antenna, heightestimates with uncertainties on the order of 200 feet are now possible.While providing useful height information at very short ranges, theheight estimates are still of limited use at further ranges. Also,volume coverage is restricted accordingly with the narrower pencil beam.The present invention seeks to overcome these limitations by providingbetter 3D localization capabilities. In particular, means are disclosedherein to provide both better height estimates (reduced heightuncertainty) and greater volume coverage.

Merrill I. Skolnik in his Introduction to Radar Systems, 2^(nd) Edition,McGraw-Hill Book Company 1980 and his Radar Handbook, 2^(nd) Edition,McGraw-Hill, Inc., 1990, describes height-finding radars that usenodding horizontal fan beams. These radars are steered to the bearingwhere targets have been detected by an independent 2D air-surveillanceradar. These height-finding radars can not get height estimates for morethan 20 or so targets per minute, and have problems withazimuth-elevation (Az-El) ambiguities in dense target environments.Military airborne and land-based tracking radars provide heightinformation for a single target only (via closed-loop steering in bothdimensions). They use monopulse or sequential lobing techniques toobtain the off-boresight error signals, but like the height-findingradars, are unable to perform 3D surveillance. Military 3D surveillanceradars, on the other hand, employ rotating phased array antennas thatform either multiple receive beams or rapidly electronic-scanning pencilbeams. See Radar Applications, Merrill I. Skolnik, IEEE Press New York,1987. Like these radar systems, the present invention is also true 3Dsurveillance; its antenna rotates in azimuth while estimating height.However, the present invention is low-cost, while military 3D radarsystems are orders of magnitude more expensive, because of their phasedarray antennas. The present invention does not use expensive phasedarrays but uses marine radars and PC-based processing to achieveconsiderable cost reduction, especially as compared to military systems.

The U.S. and Canada have conceived and are developing a North-AmericanBird Strike Advisory System (NABSAS). This system will monitor andprovide information to users on bird activity and hazards (to aircraft)at numerous sites throughout North America. It includes a network ofavian radars as part of its data sources, and bird heights as well asbird ground tracks are desired. 3D avian radars in accordance with thepresent invention will provide ideal sources of bird information forthis Advisory System.

It will be obvious to those skilled in the art that the sameimprovements described herein are applicable to low-cost radars used inother applications such as homeland security. Any radar with plotextraction (i.e. detection) could use the apparatus and method describedherein to estimate height of detected targets. Examples of such radarsare described in US Patent Application Publication No. 2006/0238406entitled “Low-cost, High-performance Radar Networks,” which isincorporated herein by reference.

OBJECTS OF THE INVENTION

It is an object of the present invention to provide improvedstate-of-the-art avian radar systems that extend current 2D targetlocalization capabilities to 3D ones.

A primary object of the current invention is to provide an affordable 3Davian radar system capable of localizing bird targets and other targetsin three dimensions (latitude, longitude, and height).

Another object of the current invention is to provide the means toaffordably upgrade existing 2D avian radar systems so that they canlocalize bird targets in 3D.

A key object of the present invention is to provide the means ofproducing significantly more accurate target height estimates, ascompared to conventional 2D avian radars, while not reducing volumecoverage.

Another object of the present invention is to provide the means ofproducing significantly greater volume coverage, as compared toconventional 2D avian radars employing dish antennas, while not reducingthe accuracy of target height estimates.

Yet another object of the present invention is to improve the accuracyof target RCS estimates.

A final object the present invention is to provide a radar system thatenables a determination as to whether a bird and an aircraft are likelyto collide.

These and other objects of the invention will be apparent from thedrawings and descriptions included herein. It is to be noted that eachobject of the invention is achieved by at least one embodiment of theinvention. However, it is not necessarily the case that every embodimentof the invention meets every object of the invention as discussedherein.

SUMMARY OF THE INVENTION

The present invention concerns practical improvements overstate-of-the-art 2D avian radar systems, including improvements inantenna designs and related and necessary radar transceivermodifications. The improvements include the following features:

-   -   Inexpensive and incremental to current systems    -   Extended height coverage    -   Improved height estimation within the covered extent    -   Low sidelobe response at ground level (zero elevation)    -   Narrow-beam azimuth response

In accordance with the present invention, the following general radarsystem designs provide (to varying degrees) the desired features listedabove:

-   -   1. A radar system whose antenna has multiple stacked pencil        beams, and that switches between them rapidly in time        (sequential lobing)    -   2. A radar system employing a monopulse antenna that receives on        multiple stacked beams simultaneously    -   3. A radar system with a single pencil beam that slowly scans up        and down in elevation, while rotating rapidly in azimuth. Such a        system will not get simultaneous height coverage and estimation,        but will get them over time.    -   4. Two single-beam radar systems operating side-by-side at        different fixed elevation angles.

For the present invention, height-finding antennas and techniques areapplied to avian radar systems in order to provide a means for providingheight information about detected bird targets for BASH and NRMapplications. The invention uses custom-designed antennas preferablyfitted to a COTS radar transceiver (although using a custom-built radartransceiver to facilitate integration still falls in the spirit of thisinvention); and novel radar signal and data processing algorithms toestimate the height of detected bird targets.

A 3D radar system comprises, in accordance with the present invention,an antenna provided with means for varying its effective pointingdirection in elevation, a radar transmitter operatively connected to theantenna for generating a radar signal for emission via the antenna, aradar receiver operatively connected to the antenna, an azimuth scanneroperatively coupled to the antenna for rotating same about an axis, anda processor operatively connected to the receiver, the processor beingconfigured for detecting and localizing airborne targets in azimuth andrange, the processor being further configured for estimating a height ofeach detected target height based on relative amplitudes of echo returnsas a function of elevation pointing direction of the antenna.

A related method of determining the heights of airborne targetscomprises, in accordance with the present invention, (a) operating aradar system to illuminate and detect the targets, the radar systemhaving at least one radar antenna, (b) during the operating of the radarsystem, varying an antenna elevation pointing angle of the radarantenna, and (c) estimating detected target heights in accordance withvariation in amplitude of echo returns as a function of antennaelevation pointing angle.

A first form of the present invention utilizes a switched-beam conceptthat has an antenna with at least two selectable radar beams pointed atdifferent elevation angles. Each beam is preferably a pencil beam withall beams having the same or similar azimuth response. The azimuthbeamwidth need not equal the elevation beamwidth, as is the case whenconventional dishes are used; different applications will have differentpreferred aspect ratios. Each beam preferably has reasonably low worstsidelobes (typically −20 dB), and has even lower ones at zero elevation(typically −25 dB or lower). The lowest beam is preferably elevatedenough that zero-elevation ground returns are in its low sidelobes; andthe lowest beam may be elevated even higher. The second beam is elevatedtypically between ½ and 2 beamwidths above the lowest one, and any otherhigher beams will have similar separation. A preferred embodiment has 1°beamwidth in azimuth, 3° beamwidth in elevation, and has the 2 beamselevated at 5° and 9°.

A desirable option is to have the actual elevation of the beamsadjustable mechanically when the radar is offline (e.g. bytilting/rotating the antenna structure to desired setting and fixing itin place). In the above example, beams at 5° and 9° could be the nominal(flat) setting, but the structure could be tilted up (i.e. adjusted) sothat they are at say 10° and 14°. An electrical control could beprovided as well so that a radar operator could effectuate thismechanical adjustment using a joystick, slider or some other convenientsoftware or hardware control interface.

A preferred embodiment of a switched-beam antenna in accordance with thepresent invention is a reflector antenna with two or more verticallystacked feed horns, each horn being a simple single-mode flaredwaveguide type. Offset feed designs may be preferred for achieving lowersidelobes (eliminating feed blockage).

The antenna preferably rotates continuously 360° in azimuth at-least 24revolutions per minute (RPM) while transmitting and receiving. It may bedesirable to have a selectable rotation rate. The rotating antenna istypically mounted near ground level; it could be on the roof of atrailer or a small building, or it could have its own dedicatedstructure. Some sites may require the antenna to be raised to 10 feet orso above ground in order to clear nearby obstructions. The rotatingantenna is usually protected from (or immune to) the environment (wind,rain, dirt, etc.); any protective measures should not significantlydistort beam patterns nor raise sidelobes above tolerable levels. Therotating antenna boresight must be (mostly) unobstructed from mechanicalapparatus; some applications may tolerate an obstructed azimuth sector.

A high-power switch, usually in the 2 kW to 60 kW range to match thepower provided by a COTS marine radar transceiver, rotates with theantenna and switches between the beams for both the transmitted and thereceived signals. The processor preferably controls the switch, and canswitch between beams on a per-pulse basis according to an arbitraryprogrammed pattern. Switching preferably occurs during the dead timebetween the longest-range return and the start of the next transmittedpulse. A rotary joint with a slip ring connection provides a path forRF, power for the switch, and switching control signals while the switchand antenna rotate in azimuth. A wireless connection, a battery, and/orsome other state-of-the-art schemes, could alternatively provide RF,power and/or control to the switch, thereby obviating the need for aspecialized rotary joint.

An alternative switched-beam implementation does not require a rotatingor a high-power switch. A low-power switch operates on only the received(Rx) signals. Transmission occurs out of both beams (or out of a thirdbeam that covers both). RF is delivered to the beams via the sum channelof a dual-channel rotary joint and a hybrid. The Rx signals from bothbeams are delivered to the switch via the hybrid, the sum and differencechannels of the dual-channel rotary joint and another hybrid. Somewhatpoorer elevation discrimination will result, because transmission willbe through both beams.

A second form of the present invention is a monopulse system, which isan alternative to a switched-beam one, with the likelihood of highersystem cost and complexity. A monopulse system transmits out of a singlebeam on every pulse, and simultaneously receives signals from twodistinct beams on every pulse. Beam shape requirements are similar tothe switched-beam concept. A monopulse system needs two receive beamsstacked in elevation, and a transmit beam that is just wide enough tocover both receive beams. Transmission occurs out of both beams (or outof a third beam that covers both). RF is delivered to them via the sumchannel of dual-channel rotary joint and through a hybrid. Monopulserequires two receive paths, each from the antenna through the samplingsystem. The received signals from both beams are delivered to thereceivers via the hybrid, the sum and difference channels of thedual-channel rotary joint and through another hybrid.

A third form of the present invention is the slow-elevation-scanningsystem, which is another alternative to switched beam. A single beam isslowly nodded up and down in elevation while it rapidly rotates inazimuth (helical scan). Nodding could be mechanical or electronic.

Nodding is slow enough that targets remain within the beam for severalconsecutive scans, long enough to form tracks. The apparatus must beable to control nodding while rotating in azimuth. Elevation coverage isnot obtained instantaneously, but over periods of a few minutes. This isthe scheme's key disadvantage: It does not detect every bird, but getsthe hourly, daily, seasonal activity (in this respect, it is like aweather radar). This scheme has some key advantages over the multi-beamsolutions. It is more flexible in the choice of coverage region (e.g.could look between 5° and 10° during day, 10° and 20° at night, etc.).It is a much simpler increment to the currently existing solutions: Theantenna is a simple conventional dish, no modifications to the receiverand sampling system are required, and the changes to the processing areconfined to the interpretation of the track data. The processor needs tobe kept informed of the azimuth (Az) and elevation (El) positions (viasignals from scanner). The processor preferably controls elevationaccording to operator-set parameters.

A fourth form of the present invention, which is an alternative to theswitched-beam system, involves using two (or more) independentsingle-beam avian radar systems operating side-by-side with theirrespective antennas set at different fixed elevation angles. Each avianradar detects and preferably tracks targets within its respectivecoverage volume, using its own receiver and processor. Detections and/ortracks from each radar are combined in a downstream fusion processor,which estimates height for each target based on its relative echoamplitudes from each of the radars.

Regardless of the form of the present invention, for a given target, itsheight estimate is based on the ratio of amplitudes received from eachbeam in the target's range-azimuth cell, at as close to the same time aspossible. Preferably, the height-estimation algorithms use interpolationto determine precisely where in elevation such a ratio would occur,thereby producing a better height estimate. The radar processor detectstargets in each beam using state-of-the-art detection methods known tothose skilled in the art and preferably tracks targets as well, usingstate-of-the-art multi-target tracking algorithms known to those skilledin the art such as those detection and tracking algorithms described inU.S. patent application Ser. No. 11/110,436 Low-cost, High-performanceRadar Networks] which are included herein by reference. A multi-targettracker is preferably included in the processor as it facilitates targettrack association (across beams) and allows for smoothing of the noisyper-detection height estimates using methods known to those skilled inthe art, thereby producing better height estimates. Various methodsknown to those skilled in the art can be used for displaying the heightof detected targets to users, including: color, intensity, and/ornumerical displays indicating the height for each target, as well asstatistical displays such as histograms which characterize heightdistribution for several or all targets.

A related advantage of having height information is that more accurateestimates of target radar cross-section (RCS) are obtainable, aiding theclassification of targets. When the radar system knows both the azimuthand elevation angles associated with a particular target, then targetamplitude can be directly converted to RCS using methods known to thoseskilled in the art. If the system does not know where the target isrelative to the (elevation) center of beam, then the target amplitudehas an unknown beam gain factor.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram of a switched-beam avian height-finding radarapparatus in accordance with the present invention.

FIG. 2 is a block diagram showing a low-power-switch version of theswitched-beam apparatus shown in FIG. 1.

FIG. 3 is a block diagram of a monopulse avian height-finding radarapparatus in accordance with the present invention.

FIG. 4 is a block diagram of a slow-elevation-scanning avianheight-finding radar apparatus in accordance with the present invention.

FIG. 5 is a block diagram of a frequency-scanning switched-beam avianheight-finding radar apparatus in accordance with the present invention.

FIG. 6 is a block diagram of a multiple, side-by-side avianheight-finding radar apparatus in accordance with the present invention.

FIG. 7 is a block diagram of the radar processor subsystems and outputdestinations.

DETAILED DESCRIPTION

A block diagram of a switched-beam avian height-finding radar apparatus1 in accordance with the present invention is shown in FIG. 1.Characteristics of each block are as follows. The avian height-findingradar apparatus 1 includes a radar transmitter 2 that is typicallynoncoherent and transmits pulses of constant width at a constant pulserepetition frequency (PRF) at X-band or S-Band (or other bands). Radarapparatus 1 typically has either a continuously rotating orsector-scanning antenna 3. Antenna 3 is typically mounted near groundlevel within (or near) the area to be monitored.

The azimuth scanner 4 rotates the antenna 3 continuously in azimuthwhile the antenna 3 is transmitting and receiving. The circulator 5,limiter 6 and receiver 7 are conventional radar components such as thosefound in marine radar transceivers. The sampling system, 8 digitizes theradar return video signal.

The switched-beam antenna 3 has (at least) 2 selectable radar beams 15pointed at different elevation angles. The high-power switch 10 rotateswith the antenna 3 and switches between the beams for both thetransmitted pulse and the received signals. The processor 11 controlsthe switch. The rotary joint with a slip ring connection 12 provides apath for RF, power for the switch, and controls switching while theswitch and antenna rotate in azimuth.

The switch control circuit 13 drives the switch 10 into its respectivestates. It preferably extracts pulse transmission timing informationfrom the RF signal 14 (or from transmitter exciter signals). It formsswitch state signals after programmed delays from the sensed RF signal,with delays and switching pattern designated by processor 11.Preferably, the switch changes state every pulse causing the beams toalternate in a pulse-to-pulse fashion.

An alternate switched-beam implementation 20 is shown in FIG. 2. Thelow-power switch 16 does not rotate with the antenna 3 and operates onthe Rx signals only. Transmission occurs out of both beams 15. RF isdelivered to them via the sum channel of dual-channel rotary joint 19and through hybrid 17. The received signals from both beams aredelivered to the switch via the hybrid 17, the sum and differencechannels of dual-channel rotary joint 19 and through hybrid 18.

A monopulse avian height-finding radar apparatus 21 shown in FIG. 3 isan alternative to switched-beam ones. RF pulses are delivered to bothbeams 15 via the sum channel of dual-channel rotary joint 19 and throughhybrid 17. The two receive paths (L and U) 22, each run from the antenna3 through to the sampling system 8. The received signals from both beams15 are delivered to the receivers via the hybrid 17, the sum anddifference channels of dual-channel rotary joint 19 and through hybrid18.

The slow-elevation-scanning avian height-finding radar apparatus 24shown in FIG. 4 is another alternative in accordance with the presentinvention. The antenna 3 is simpler than the above designs, with only asingle beam. The Az-El scanner 23 moves the antenna 3 through itshelical scan. The Elevation Rotary Joint 25 and Azimuth Rotary Joint 12allow RF transmission while scanning in both dimensions.

Scan-to-Scan Elevation Switching is an alternative mode for aswitched-beam system. The antenna remains at one elevation setting forone scan, is switched to the other for the next scan, and then back,etc. This doubles the revisit time for targets only visible in one beam,meaning a reduction in tracking performance. This solution could be usedif a switched beam antenna was available, but switching takes too longto apply it on alternate pulses (for example, in the case of amechanical switch). The processor would analyze the alternatingvariation in amplitude over several scans in order to derive height forany track. The tracker must be set to handle targets that are onlydetected in every other scan, which will happen for those at heights notwithin both beams. The system could also be configured to mimic slowelevation scanning, i.e. spend several consecutive scans at oneelevation setting, then switching to the next, etc.

The frequency-scanning apparatus 26 shown in FIG. 5 is an alternativeswitched-beam system, where tuning of the transceiver RF (from pulse topulse) scans the beam in elevation, giving continuously selectable beampositions. This gives much flexibility in the operator's control ofelevation coverage. The apparatus employs a flat-panel frequency-scannedphased-array antenna 27. Such an antenna delivers phased-arrayperformance without the need for phase shifters, at much reduced cost.Lower sidelobes (than typical reflectors) can be achieved by carefuldesign of the aperture taper. The radar transmitter 2 and receiver 7must be rapidly tunable over a fairly wide bandwidth, which prevents theapparatus from using inexpensive COTS marine radars.

An alternative height-finding avian radar system 28 shown in FIG. 6consists of two (or more) side-by-side avian radars, where one radarsubsystem 29 operates at a lower elevation angle, the other radarsubsystem 30 at higher one. Each radar subsystem 29 and 30 has its ownreceiver 7, sampling system 8 and processor 11. Tracks (or detections)are combined in fusion processor 31, which then derives height estimatesfor detected targets.

Other scanning alternatives are possible, but the above are more suitedto avian radars, where 360° azimuth coverage is usually required. Onecould scan quickly mechanically up-and-down (or around) in elevationwhile rotating slower in azimuth. One could scan in 2D in aback-and-forth raster mode (electronic, mechanical, or both). While aphased-array antenna could be integrated into the radar sensor of thepresent invention, it is not a preferred embodiment of the presentinvention due to the significantly higher cost anticipated for such anantenna.

Preferably, embodiments of a radar system as disclosed herein aim totake advantage of standardized COTS technologies to the maximum extentpossible in order to keep the system cost low and to provide for lowlife cycle costs associated with maintainability, upgrade ability andtraining. Preferably, COTS marine radars are used as the radar sensor inorder to minimize sensor costs. The radar processor 11 itselfincorporates sophisticated algorithms and software that runs on COTSpersonal computers (PC). Preferred embodiments provide a low-cost,high-performance, land-based radar sensor designed for avian radarapplications. Preferred embodiments digitize the raw radar video signalfrom the marine radar receiver and use a PC-based radar processor withsophisticated processing such as the detection, tracking and displayprocessing described in US Patent Application Publication No.2006/0238406 entitled “Low-cost, High-performance Radar Networks,” whichis incorporated herein by reference and further described below.

The radar processor 11 shown in FIG. 7 preferably incorporates adetection processor 32, a track processor 33, a post-processor 34 and adisplay processor 35. The detection processor 32 performs radar signalprocessing functions known to those skilled in the art such asscan-conversion, clutter suppression through the use of adaptiveclutter-map processing to remove ground and weather clutter, sectorblanking to suppress detections and interference in regions that are notof interest, adaptive thresholding such as constant false alarm rate(CFAR) processing, and digital sensitivity time control (STC). Thedetection processor declares the presence and location of target plots36 preferably on each radar scan. The information on each plotpreferably includes time, range, azimuth, elevation (beam center), andamplitude. The track processor 33 sorts the time-series of detections(also called plots) into either target tracks 37 (confirmed targets withestimated dynamics) or false alarms. The information on each trackedtarget preferably includes time and estimated 3D spatial position,velocity, and RCS.

A plot-to-track association algorithm provides means to resolveambiguities produced by multiple targets, missed detections, falsealarms, and maneuvering targets, whereas a track filtering algorithmprovides high quality estimates of target dynamics for the associationalgorithms and for the display processor. The track processor preferablyuses a sophisticated plot-to-track association algorithm called MHT andpreferably uses an advanced track-filtering algorithm called InteractingMultiple Model (IMM) filtering as described in US Patent ApplicationPublication No. 2006/0238406.

For the apparatus shown in FIG. 1, FIG. 2, FIG. 3, FIG. 4 and FIG. 5,processor 11 also includes the height-finding algorithms in accordancewith the present invention. For the side-by-side apparatus in FIG. 6, aseparate fusion processor 31 performs the height-finding calculations toproduce target height estimates.

The post-processor 34 (FIG. 7) analyzes the tracks data 37 andpreferably distributes target data 38 to either a network 39 or fusionprocessor 31. Post-processor 34 can also send real-time target data 38to a local display processor 35, which displays tracks in real-time onan operator's monitor. The target data 38 consists of tracks data 37refined into user-specific products such as alerts, statisticalsummaries, reduced subsets, etc.

The height-finding algorithms in accordance with the present invention,for a given target, are based on the ratio of amplitudes received fromeach beam in the target's range-azimuth cell, at as close to the sametime as possible. Antenna calibration data (previously acquired) areused to translate the target amplitude ratio to an estimate of thetarget elevation angle, which can then be translated to a heightestimate through simple geometry. Preferably, the height-estimationalgorithms use interpolation to determine precisely where in elevationsuch a ratio would occur, thereby producing a better height estimate.Some nonlinear function of amplitude could also be used in place ofamplitude. The elevation beam pattern for each beam of the antenna needsto be calibrated, or alternatively, the ratio itself. Any antennacalibration method known-to-those skilled in the art may be used togenerate the required calibration data and table look-up methods knownto those skilled in the art may be used to directly provide heightestimates. The radar processor 11 detects targets in each beam usingstate-of-the-art detection methods known to those skilled in the art andpreferably tracks targets as well, using state-of-the-art multi-targettracking algorithms known to those skilled in the art such as thosedetection and tracking algorithms described in US Patent ApplicationPublication No. 2006/0238406, which are included herein by reference.

A multi-target tracker, such as the aforementioned MHT/IMM automaticmulti-target tracker which is ideal for surveillance tracking with manytargets, is preferably included in the processor 11 as it facilitatestarget track association (across beams) and allows for smoothing of thenoisy per-detection height estimates using methods known to thoseskilled in the art, thereby producing better height estimates. Considerthe case where the antenna switches between two elevation beams everypulse. For each full azimuth scan (revolution) of the antenna, two scanmatrices of radar echo data are produced, one for each of the two beams.Detections are automatically computed for each of the scan matrices, andthis process is repeated continuously from scan to scan. Each detectionincludes a location (e.g. range/azimuth) and an amplitude (or somenonlinear function of amplitude). Without a multi-target tracker,determining which detections from the first scan matrix are associatedwith which detections in the second scan matrix (i.e. arise from thesame respective targets) is a very difficult task. This is so becausedetections are inherently noisy and false alarms confuse the situation.This is even more the situation when low detection thresholds are usedto improve detection sensitivity as is done in US Patent ApplicationPublication No. 2006/0238406. As a result, averaging the resultingamplitudes (or ratios) over multiple scans does not perform as well asone would hope due to incorrect associations. With a multi-targettracker operating independently on each of the scan matrices over time,high-quality confirmed tracks result. For each target, its track willpreferably record the amplitude from each detection used in theformation of that track, and preferably smooth the sequence ofamplitudes to form a more accurate target amplitude estimate within thatparticular beam. Now track-to-track association methods known to thoseskilled in the art can be used across the beams to associate tracksresulting from the series of first scan matrices with those resultingfrom the series of second scan matrices that belong to the samerespective targets.

Finally, the ratio of amplitudes can preferably be computed on ascan-by-scan basis from the smoothed amplitude estimates from associatedtrack pairs in order to compute a series of height estimates that areeffectively smoothed over multiple scans, thereby resulting in morerobust and more accurate height estimates. Alternative smoothingtechniques are to smooth the per-scan height estimates or the per-scanamplitude ratios, but these methods tend to be less robust tointerference and missed detections.

A related advantage of having good target height (or equivalentlyelevation angle) information is that more accurate estimates of targetradar cross-section (RCS) are obtainable. RCS is a property of a target;however, it is estimated using target echo amplitude. Target echoamplitude is dependent on the two-way beam pattern, which can becharacterized as having a gain in the azimuth direction and a gain inthe elevation dimension. When the radar system knows both the azimuthand elevation angles associated with a particular target as in thepresent invention, then target amplitude can be directly converted toRCS using radar equation and beam pattern calibration methods known tothose skilled in the art. If the system does not know where the targetis relative to the (elevation) center of beam, then the target amplitudehas an unknown beam gain factor, making a good target RCS estimateimpossible.

Good RCS estimates can lead to the ability to better classify differentclasses of targets. For example, an eagle will have a larger RCS than asparrow. Improving the quality of RCS estimates will ultimately improveone's ability to use these estimates along with other radardescriminants to classify targets.

The processed information produced by radar processor can be presentedto the operator on a local real-time display. This information mayinclude scan-converted video, target data including detection data (withtime history) and track data, maps, user data (e.g. text, push pins)etc. Preferred embodiments have radar target data geo-referenced using ageographic information system (GIS) so that target data are tagged toearth co-ordinates. Preferably, a map is integrated with the radardisplay and provides a background on which is overlaid geo-referencedradar data.

The track data produced by preferred embodiments contains detailed (butcompact) long-term behavior information on individual targets. For anygiven scenario, these data can be automatically tested for hazardousactivity, in order to generate alerts. Because the information isdetailed, alerts can reflect complex behavior, such as origins anddestinations of birds, runway approaches, density, etc. Targetdetection, tracking and hazard recognition algorithms may be customizedfor specific hazards and scenarios. Alerts can include an audible alarmand display indication to an operator, or a transmitted message to aremote user. The low-bandwidth track and alert information can be easilysent to central locations, and directly to end users, providingeconomical, effective monitoring. Automated alerts may be sent to remoteusers who require them. This enables the radar surveillance system torun unattended with users alerted only when necessary. Furthermore,track displays can be provided to remote users to give them a clearpicture of the situation when alerts arise. The system preferablyexploits COTS communication technology to provide such remote alerts anddisplays inexpensively.

Many of the aforementioned radar processor features as well as featuresnot mentioned above are described in the articles Low-cost RadarSurveillance of Inland Waterways for Homeland Security Applications,Weber, P et al., 2004 IEEE Radar Conference, Apr. 26-29, 2004,Philadelphia, Pa., and Affordable Avian Radar Surveillance Systems forNatural Resource Management and BASH Applications, Nohara, T J et al,2005 IEEE International Radar Conference, May 9-12, 2005, Arlington, Va.and US Patent Application Publication No. 2006/0238406, all of which areincorporated herein by reference.

For avian radar applications, one radar system, or even severalindependently operating radar systems are often not enough to provide ahigh-performance, composite picture covering the area of interest. Forany single radar, there are gaps in coverage due to obstructions, andthe area covered may not be a wide enough. One or more radar sensorapparatuses can be connected to a network to distribute their compositeinformation to remote users. Since the target data contain all of theimportant target information (date, time, position including height inaccordance with the present invention, dynamics, plot size, intensity,etc.), remote situational awareness is easily realized. Radar systems asdisclosed herein may be networked to a central monitoring station (CMS).In that case, the CMS has a fusion/display processor that processes,integrates (and/or fuses), displays and archives the data. In additionto monitoring live radar data, the CMS also provides the capability toplay back past recorded radar data. Some of the performance improvementsachievable through integration and fusion of data from radar networksinclude:

-   -   Spatial diversity against target fluctuations in RCS (necessary        for small targets)    -   Spatial diversity for shadowing due to geographic obstructions

A recorder can store the target data including track data and detectiondata. Target data can easily be stored continuously, 24/7, withoutstressing the storage capacity of a COTS PC. These same data can bedistributed over a network. The stored data can subsequently be playedback through any computer running the radar processor software; it isnot necessary that it be connected to a radar apparatus. This feature isuseful for off-line analysis. Target data can be archived forlonger-term investigations. The recorder supports continuous writing oftarget data directly to a database (as well as to other file formats).The database can reside locally on the radar processor computer, onanother computer on the network, or on both. The database is usedpreferably for post-processing, for interaction with externalgeographical information systems (GIS) systems, for remote radardisplays, for support for web services, and for further research anddevelopment (e.g. to investigate and develop target identificationalgorithms).

The applications towards which the present invention is directed requirefurther research and development (R&D) in order to increase andestablish knowledge concerning target behavior. This knowledge can beused, for example, for automatic target identification. Off-lineanalysis of target data can be used with ground-truth data to betterunderstand bird signatures, which could then be used to develop birdidentification algorithms. In BASH applications, knowing the kind ofbird that is being tracked is valuable for forming an appropriateresponse (e.g. should aircraft delay take-offs and landings or make anevasive maneuver to increase safety). Databases can continuously storecomplete target detection and track data over extended periods of timein order to support such R&D activities. One can rapidly play backstored target data into the radar processor in order to study andanalyze the data.

Particular features of our invention have been described herein.However, simple variations and extensions known to those skilled in theart are certainly within the scope and spirit of the present invention.This includes variations on integration of the functional blocksdescribed herein. For example, the sampling system 8 could be integratedwith the processor 11 forming a single functional unit, withoutdeparting from the spirit of the invention.

What is claimed is:
 1. A 3D radar surveillance system for simultaneouslytracking multiple targets, comprising: an antenna; means operativelyconnected to said antenna for varying an effective elevation pointingdirection of said antenna; a radar transmitter operatively connected tosaid antenna, said transmitter generating a radar signal for emissionvia said antenna; a exactly one radar receiver operatively connected tosaid antenna; an azimuth scanner operatively coupled to said antenna forrotating said antenna about an axis during repeated azimuth scans, saidreceiver being configured to receive echo returns from all transmittedradar signals for all azimuth directions and effective elevationpointing directions of said antenna; and a processor operativelyconnected to said receiver, said processor being configured forcontemporaneously detecting and localizing multiple airborne targets inazimuth and range for each of said azimuth scans, said processor beingfurther configured for estimating a height above a ground surface ofeach detected an localized target based on relative, the estimatedheight being geometrically related to a respective, estimated targetelevation angle, said processor being further configured for computingsaid estimated target elevation angle by mathematically combiningamplitudes of temporally spaced echo returns as a function of from suchdetected and localized targets in response to respective temporallyspaced transmit radar signals received at a minimum of two respectiveelevation pointing direction of said antenna.
 2. The system defined inclaim 1 wherein said antenna includes means for generating at least twobeams and said means for varying includes means for selecting a givenbeam for a given radar pulse.
 3. The system defined in claim 2 whereinsaid means for generating and said means for selecting includes ahigh-power RF switch that rotates with said antenna about said axis,said switch being operable to cause (a) said radar pulse to betransmitted via said given beam and (b) pulse echo returns associatedwith said radar pulse to be received via said given beam.
 4. The systemdefined in. claim 2 wherein said means for selecting includes alow-power RF switch that is stationary relative to said axis and doesnot rotate with said antenna, said switch being operable to direct pulseecho returns from said given beam to said receiver.
 5. The systemdefined in claim 2 wherein said antenna is a reflector, said means forgenerating including multiple feeds.
 6. The system defined in claim 2wherein said antenna is a frequency-scanning antenna and said means forgenerating includes a variable-frequency transceiver that is tuned togenerate said at least two beams.
 7. The system defined in claim 2wherein said antenna is a phased-array antenna and said means forgenerating and said means for selecting include a beam forming network.8. The system defined in claim 2 wherein said antenna and said means forgenerating include two antennas oriented to provide respective beamswith respective elevation pointing directions different from oneanother.
 9. The system defined in claim 2 wherein said at least twobeams are vertically stacked pencil beams and said means for varyingincludes an RF switch.
 10. The system defined in claim 1 wherein saidantenna is an elevation monopulse antenna and said radar receiverincludes a dedicated receiver for each of a plurality of antenna receivechannels, said means for varying including means for selecting fromamong the antenna receive channels for a given radar pulse.
 11. Thesystem defined in claim 10 1 wherein said antenna is a reflector withmultiple feeds.
 12. The system defined in claim 10 wherein the dedicatedreceivers are non-coherent and the associated receive channels are fromthe upper and lower beams of said antenna.
 13. The system defined inclaim 1 wherein said radar transmitter and said radar receiver arenoncoherent.
 14. The system defined in claim 1 wherein said radartransmitter and said radar receiver are from a COTS marine radar. 15.The system defined in claim 1 wherein said receiver has a digitizedoutput.
 16. The system defined in claim 1 wherein said means for varyingincludes an elevation scanner.
 17. The system defined in claim 1 whereinsaid processor is a COTS PC.
 18. The system defined in claim 1 wheresaid processor executes integration, interference suppression, cluttersuppression, and adaptive thresholding.
 19. The system defined in claim1 wherein said means for varying an effective elevation pointingdirection includes components taken from the group consisting ofmechanical components and electrical components.
 20. The system definedin claim 19 wherein said means for varying an effective elevationpointing direction includes components taken from the group consistingof a plurality of vertically stacked feed horns, an RF switch forswitching among a plurality of beams of different fixed elevationangles, a rotary joint for conveying signals from said antenna duringrotation thereof in elevation, a dual-channel rotary joint and a hybrid,frequency-scanning apparatus, a beam forming network, a plurality ofreceive channels, a channel selector, and an elevation scanner.
 21. Thesystem defined in claim 1 wherein said means for varying an effectiveelevation pointing direction are means for varying the effectiveelevation pointing direction of said antenna while said antenna rotatesabout said axis.
 22. A 3D radar surveillance method of contemporaneouslydetermining the heights of multiple airborne targets above a groundsurface, comprising: operating a radar system over successive azimuthscans to illuminate and detect the targets in a search volume includinga plurality of range-azimuth cells, said radar system having a at leastone radar antenna; during the operating of said radar system, varying anantenna elevation pointing angle of said radar antenna; and detectingand localizing multiple targets on a plane in respective range-azimuthcells, and estimating the height above a ground surface for eachdetected and localized target from, the estimated height beinggeometrically related to a respective, estimated target elevation angle,the estimating of said height including computing a value for saidestimated target elevation angle by mathematically combining theamplitudes of temporally spaced echo returns thereof received from saidradar antenna pointed along at least two different elevation pointingangles, the estimating of the height including computing heightestimates from relative amplitudes of said echo returns as a function ofsaid elevation pointing angles from such detected and localized targetin response to respective temporally spaced illumination signalsreceived at a minimum of two respective elevation pointing directions ofsaid antenna.
 23. The method defined in claim 22 wherein said varyingcomprises emitting a plurality of beams via said antenna.
 24. The methoddefined in claim 23 wherein radar transmission and reception isalternated between said beams from pulse to pulse.
 25. The methoddefined in claim 23 wherein radar transmission and reception is throughall of said beams on every transmission pulse.
 26. The method defined inclaim 23 where said beams are vertically stacked pencil beams.
 27. Themethod defined in claim 22 wherein said varying comprises operating anelevation scanner.
 28. The method defined in claim 22 wherein said radarsystem includes at least two radar subsystems proximate to one another,said varying comprising operating said at least two radar subsystems sothat each radar subsystem is pointed at a different elevation angle. 29.The method defined in claim 22 further comprising operating a processorto track the detected airborne targets.
 30. The method defined in claim22 wherein said estimating includes interpolating in elevation.
 31. Themethod defined in claim 22 wherein said estimating includes using targettracks in an association process to identify tracks in different beamsbelonging to a common target, thereby enabling a smoothing and improvingof height estimates.
 32. The method defined in claim 22 furthercomprising using the height estimates further to estimate target radarcross-section.
 33. The method defined in. claim 22 further comprisingusing the height estimates further for target classification.
 34. Themethod defined in claim 22 further comprising distributing heightinformation to a network.
 35. The method defined in claim 22 furthercomprising automatically notifying or alerting users of hazards orsituations of interest.
 36. The method defined in claim 22 furthercomprising combining target height and range estimates and radar echointensities to form accurate estimates of radar cross-sections.
 37. Themethod defined in claim 22 farther comprising continually updatingestimated dynamics vectors, including speed, heading, position, andheight for each of said targets.
 38. The method defined in claim 22wherein said estimating is carried out for all targets detected in saidsearch volume, to provide 3D localization of such targets.
 39. Themethod defined in claim 22 wherein the operating of said radar system iscontinuous so that the search volume is scanned repeatedly at regulartime intervals, causing targets to be repeatedly illuminated anddetected.
 40. The method defined in. claim 22 further comprisingrotating or scanning said radar antenna in azimuth.
 41. The systemdefined in claim 1 wherein an antenna beam is associated with each ofsaid elevation pointing directions, and wherein said processor includesa multi-target tracker that is configured to track each target for eachbeam and identify associated tracks across beams for each target. 42.The system defined in claim 41 wherein said processor is configured torecord, for each target track, an amplitude from each detection used inthe formation of such track, said processor being further configured toperform at least one additional operation taken from the group of (i)smoothing such amplitudes to form a more accurate target amplitudeestimate within each beam, and (ii) smoothing noisy per detection heightestimates over multiple scans using said associated tracks to therebyproduce better height estimates.
 43. The system defined in claim 1wherein said processor is further configured to compute a radar crosssection estimate for each target using the estimated target elevationangle to improve the radar cross section estimate.
 44. The methoddefined in claim 22 wherein the varying of said antenna elevationpointing angle means steering its associated beam to said elevationpointing angle, and wherein the detecting and localizing multipletargets includes performing multi-target tracking on detected targets togenerate target tracks for each beam, and associating tracks acrossbeams belonging to the same target.
 45. The method defined in claim 44wherein for each target track, an amplitude from each detection used inthe formation of such track is recorded, and where at least oneadditional operation is performed taken from the group of (i) smoothingsuch amplitudes to form a more accurate target amplitude estimate for agiven beam, and (ii) smoothing noisy, per detection height estimatesover multiple scans using said associated tracks to produce betterheight estimates.
 46. The method defined in claim 22, further comprisingcomputing a radar cross section estimate for each target using theestimated target elevation angle to improve the radar cross sectionestimate.
 47. A 3D radar surveillance system comprising: an antennaprovided with means for varying its effective pointing direction inelevation to enable multiple elevation beams; a radar transmitteroperatively connected to said antenna for generating a radar signal foremission via said antenna and over said beams; a radar receiveroperatively connected to said antenna; an azimuth scanner operativelycoupled to said antenna for rotating same about an azimuth axis forrepeated scans; and a processor operatively connected to said receiver,said processor being configured for contemporaneously estimating aheight above a ground surface of each of a plurality of detected targetslocalized in azimuth and range, wherein the height estimate for anygiven detected target is geometrically related to a respective,estimated target elevation angle, said processor being configured tocompute said respective estimated target elevation angle bymathematically combining amplitudes of echo returns from such givendetected target received from at least two respective elevation beams ofsaid antenna, said processor including a multi-target tracker thattracks target detections in each beam retaining their echo returnamplitudes, said processor further configured to determine which ofthose tracks across said beams belong to the same target in order toidentify respective target relative amplitude pairs for said estimatingof target heights.
 48. The system defined in claim 47 where saidprocessor is further configured to reduce the noise associated withheight estimates computed for each detected target by smoothing therelative amplitudes retained in said tracks over multiple scans.
 49. Thesystem defined in claim 47 where said processor is further configured tocompute radar cross section estimates for each target utilizing thetarget elevation estimate to improve said radar cross section estimate.50. A radar surveillance method of contemporaneously determining theheights of multiple airborne targets above a ground surface, comprising:operating a radar system over successive scans to illuminate and detectthe targets in a search volume including a plurality of range-azimuthcells, said radar system having at least one radar antenna; during theoperating of said radar system, forming multiple elevation beams of saidat least one radar antenna centered at respective elevation pointingangles; and detecting and localizing airborne targets in saidrange-azimuth cells, estimating a height above a ground surface of eachdetected and localized target, said height being geometrically relatedto a respective, estimated target elevation angle whose value iscomputed by mathematically combining relative amplitudes of echo returnsfrom such detected and localized target received from at least tworespective elevation beams of said antenna, said estimating of targetheight further including tracking target detections from said targets ineach beam, retaining their respective echo return amplitudes anddetermining those tracks across said beams belonging to the same targetin order to identify respective target relative amplitude pairs for saidestimating of target heights.
 51. The method defined in claim 50 wheresaid estimating of target height further includes reducing the noiseassociated with height estimates computed for each detected target bysmoothing the relative amplitudes retained in said tracks over multiplescans.
 52. The method defined in claim 50 wherein said target elevationestimate is further used to estimate radar cross section for saidtarget.